Warmer and drier conditions have slowly degraded water quality in Island Park Reservoir over the last 50 years.
In the 1970s and 1980s, federal reports and university research indicate that the reservoir had relatively good water quality. Island Park Reservoir was mesotrophic—with mostly clear water and occasionally low dissolved oxygen at the reservoir’s deepest depths during the summertime.
But, as time went on, water quality began to decline. Historical satellite imagery analyzed by the U.S. Geological Survey (USGS) recently classified Island Park Reservoir as eutrophic in the 1990s—warmer water, more algal scum, poor visibility, and low deepwater oxygen levels occurring annually each summer.
Unfortunately, the decline has continued. Using HFF monitoring data from 2015–2025 and funding from Friends of Reservoirs, consultants Hazen and Sawyer determined that Island Park Reservoir is now hyper-eutrophic—warm, nutrient dense, dominated by algae, creating exceptionally high turbidity and persistently low deepwater oxygen levels. With warmer air temperatures and less streamflow into the lake, Island Park Reservoir has become a metaphorical stewpot in the heat of summer.
After 50 years of decline, summertime water quality in Island Park Reservoir is now a major limiting factor in the Henry’s Fork watershed.
Poor summertime water quality minimizes fish habitat in Island Park Reservoir, impedes hydropower production at Island Park Dam, and diminishes the fishing experience on the Henry’s Fork downstream.
Decades of decline motivate HFF’s DIRTT Plan. But, thankfully, a better future does exist. Using federal WaterSMART funds and a $100K matching grant from The Chrysanta Foundation, HFF is hiring engineering consultants to investigate the most effective solutions portfolio to address decades of water quality decline in Island Park Reservoir.
This article was originally published in HFF’s Summer 2026 Newsletter.
Our Aquatic Ecology Program Manager, Dr. Jack McLaren, joined Idaho Falls city and FIPS-Mouche leadership at the opening ceremony parade along the Snake River Greenbelt to welcome competitors to the watershed and to make sure conservation was front and center.
Drone photo of Island Park Dam taken on September 9, 2026 Looking Back on September September brought lows and highs to Island Park. Island Park Reservoir started September at 29% full and quickly fell to a low of 12% full on September 19. The rapid drawdown of the reservoir was the result of managers needing to move water downstream to American Falls (to irrigate farms in the Magic Valley with senior water rights). Such low
This story was written by Audrey Hunter (2026 intern) and was originally published in the Fall 2026 Newsletter. It’s no secret that late summers on Island Park Reservoir haven’t been in perfect conditions in recent years. Warm, turbid, algae-filled water plague reservoir recreationists while diminishing water quality downstream. Cyclical algal blooms decrease water clarity and create less-than-ideal circumstances for the reservoir’s flora and fauna. What’s the solution? HFF’s DIRTT Plan. In-Reservoir Oxygenation Nutrients released from
Drone photo taken on September 9, 2026 The Henry’s Fork Drought Management Planning Committee met on Wednesday, September 16 2026 to develop an updated reservoir management strategy. Downstream of the Henry’s Fork watershed, lower-valley irrigation demand over the rest of the season is anticipated to be slightly lower than projected last week, so Island Park Reservoir and Grassy Lake (in the Fall River drainage) do not need to deliver any more water out of the
